How can I view my scene using the Oculus Rift CV1 and the OpenHMD branch of Blender?
Assuming you are using Windows:

The way to enable this is to switch the Oculus in Extended Mode, this is done by running a specific tool that Oculus delivers with the driver (example is https://www.reddit.com/r/oculus/comments/4nvy7o/rift_dk2_possible_to_activate_extended_mode/).
Opening the HMD window in Blender should start the display in extended mode, if you can not view your desktop through the Oculus before moving the window, It is not activated correctly.

When using Linux:

If you are running Intel or AMD, this should work fine out of the box, with Nvidia, if you have recent drivers, you may need to enable Extended mode by adding AllowHMD in your Xorg, instructions can be found here: http://us.download.nvidia.com/XFree86/Linux-x86/378.09/README/xconfigoptions.html